A preferred embodiment of the invention is directed to mitigating the formation of coke deposits in petroleum refinery reactor units, particularly in the cyclones of fluidized bed coking units (fluid cokers) and reactor overheads of fluid catalytic cracking units.
Fluidized bed coking (fluid coking) is a petroleum refining process in which mixtures of heavy petroleum fractions, typically the non-distillable residue (resid) from fractionation, are converted to lighter, more useful products by thermal decomposition (coking) at elevated reaction temperatures, typically about 900 to 1100xc2x0 F. (about 480 to 590xc2x0 C.). A large vessel of coke particles maintained at the reaction temperature is fluidized with steam. The feed is heated to a pumpable temperature, mixed with atomizing steam, and fed through a plurality of feed nozzles to the fluidized bed reactor. The light hydrocarbon products of the coking reaction are vaporized, mixed with the fluidizing steam and pass upwardly through the fluidized bed into a dilute phase zone above the dense fluidized bed of coke particles. The transition between the dense bed (dense phase zone) and dilute phase, where product vapor is substantially separated from solid particles, is hereinafter referred to as the phase transition zone. The remainder of the feed liquid coats the coke particles and subsequently decomposes into layers of solid coke and lighter products which evolve as gas or vaporized liquid. The solid coke consists mainly of carbon with lesser amounts of hydrogen, sulfur, nitrogen, and traces of vanadium, nickel, iron, and other elements. The fluidized coke is circulated through a burner, where part of the coke is burned with air to raise its temperature from about 900xc2x0 F. to about 1300xc2x0 F. (about 480 to 704xc2x0 C.), and back to the fluidized bed reaction zone.
The mixture of vaporized hydrocarbon products and steam continues to flow upwardly through the dilute phase at superficial velocities of about 3 to 6 feet per second (about 1 to 2 meters per second), entraining some fine solid particles. Most of the entrained solids are separated from the gas phase by centrifugal force in one or more cyclone separators, and are returned to the dense fluidized bed by gravity. The gas phase undergoes pressure drop and cooling as it passes through the cyclone separators, primarily at the inlet and outlet passages where the velocity is increased. The cooling which accompanies the pressure decrease causes condensation of some liquid which deposits on surfaces of the cyclone inlet and outlet. Because the temperature of the liquid so condensed and deposited is higher than about 900xc2x0 F. (about 480xc2x0 C.), coking reactions occur there, leaving solid deposits of coke. Coke deposits also form on the reactor stripper sheds, and other surfaces of the fluid coker reactor.
The mixture of steam and hydrocarbon vapor is subsequently discharged from the cyclone outlet and quenched to about 750xc2x0 F. (about 400xc2x0 C.) by contact with downflowing liquid in a scrubber vessel section of the fluid coker equipped with internal sheds to facilitate contacting. A pumparound loop circulates condensed liquid to an external cooling means and back to the top row of scrubber sheds to provide cooling for the quench and condensation of the heaviest fraction of the liquid product. This heavy fraction is typically recycled to extinction by feeding back to the fluidized bed reaction zone, but may be present for several hours in the pool at the bottom of the scrubber vessel and the pumparound loop, allowing time for coke to form and deposit on shed surfaces because of the elevated temperatures.
Feed is injected through nozzles with atomizing steam into the fluidized bed reactor. The feed components not immediately vaporized coat the coke particles and are subsequently decomposed into layers of solid coke and lighter products which evolve as gas or vaporized liquids. During this conversion process some coke particles may become unevenly or too heavily coated with feed and during collision with other coke particles stick together. These agglomerated, now heavier, coke particles may not be efficiently fluidized by the steam injected into the bottom of stripper section and are subsequently carried under from the reactor section to the stripper section where they adhere to and build up on the top rows of sheds in the stripper section. Build up of deposits on the stripper sheds can become so severe due to overlapping of the deposits on adjacent sheds as to restrict fluidization of the coke in the reactor section above and eventually shut the unit down.
Fouling of cyclone outlets and scrubber sheds in a Fluid Coker results in decreased capacity and run length of the unit, culminating in costly unplanned shutdowns. The deposits are sometimes removed from the outlet of the cyclone with metal rods and water jets at high pressure to clear the cyclone outlet area and to keep the unit running. The effectiveness of this approach is temporary and unpredictable. Chunks of coke may fall back into the cyclone body and interfere with cyclone operation. The coke deposits must similarly be removed from the reactor scrubber sheds, reactor walls and other areas of the fluid coker that become fouled. It is well known in the art that providing sufficient cooling of the pumparound loop will minimize fouling of scrubber sheds, but this technique does not affect the cyclone outlet area.
Fluid Catalytic Cracking (FCC) is another petroleum refining conversion process in which heavy oil, typically the highest boiling distillable fraction, is converted to gasoline, diesel and jet fuel, heating oil, liquefied petroleum gas (LPG), chemical feedstocks, and refinery fuel gas by catalytic decomposition at similarly elevated temperatures of about 900 to 1100xc2x0 F. (about 480 to 590xc2x0 C.). In a Fluid Catalytic Cracking Unit (FCCU), the heavy oil feed is typically mixed with steam and sprayed into a rising stream of hot (1100 to 1400xc2x0 F. or about 590 to 760xc2x0 C.) powdered silica-alumina catalyst. The feed is vaporized by contact with the hot catalyst, and the vapor decomposes catalytically into the desired products within a few seconds, whereupon the solid catalyst particles are separated centrifugally from the vapor by means of cyclone separators or equivalent means. The product vapor passes through the cyclone outlets into a plenum chamber at the top of the reactor, through a discharge nozzle into an overhead line, then to a fractionator where the vapor is quenched and condensed in a zone similar the coker scrubber described above. The separated catalyst is introduced into a stripping zone in which it is further stripped with steam to recover entrained vapor. Because the stripping steam is typically at a significantly lower temperature than the spent catalyst, the catalyst is cooled by the stripping steam to a temperature significantly below the reaction temperature.
Run length or capacity of an FCCU may likewise be limited by deposition of coke in the stripper, reactor overhead, plenum, nozzle, transfer line, or inlet to the fractionator. Coke formation occurs where heat loss allows condensation of heavy hydrocarbons which decompose to form coke. Deposit formation is further aggravated by entrapment of entrained catalyst particles in the condensate. Deposits are most likely to occur where flanges or other heat sinks provide surfaces below the dew point of the product vapor. Deposits may also form at the inlet to the fractionator where expansion cooling of the hot product vapor causes condensation and subsequent coke formation at the entrance to the fractionator. The coke buildup restricts flow and increases pressure drop between the reactor overhead and the fractionator. In units limited by compressor capacity, the pressure drop may be sufficient to limit capacity long before the end of the run, and may ultimately require premature or unplanned shutdown.
In both fluid coking and fluid catalytic cracking units, there is a reaction zone in which the product vapor is in intimate contact with particulate solids, known as the dense phase zone, and a dilute phase zone in which the solids have been substantially separated from the product vapor or where coke and catalyst have disengaged from the dilute (vapor) phase. The mass of solid particles in the reaction zone is many times the mass of the product vapor, and in both types of units the heaviest reaction products condense on the solid particles to form coke. The dew point of the product vapor emerging from the reaction zone (dense phase zone) into the dilute phase zone is essentially the same temperature as the temperature at the transition from the reaction zone (dense phase zone) to the dilute phase zone known as the phase transition temperature. In many FCCU""s the reaction zone is terminated by cyclone separators, and the dilute phase is the zone into which the cyclone outlets discharge, typically a plenum at the top of the vessel housing the cyclones.
What is needed in the art is an efficient, predictable, and effective way to mitigate the formation of detrimental coke deposits in the dilute phase overhead equipment such as fluid coker cyclones and accompanying surfaces and in the overhead dilute phase zone, plenum, discharge nozzle and overhead lines of fluid catalytic cracking units to avoid loss of capacity and expensive shutdowns.
An embodiment of the present invention is a method for mitigating the condensation of liquid hydrocarbons and subsequent coke formation at temperatures in the range of about 700xc2x0 F. to 1100xc2x0 F. (about 370xc2x0 C. to 590xc2x0 C.) in the dilute phase zones of fluid cokers and FCCU""s such as occurs in overhead equipment.
An embodiment of the invention is directed to a method for mitigating the condensation of liquid hydrocarbons and subsequent coke deposition in refinery reactor units wherein during operation of said units a dense phase zone comprising hydrocarbon feed and a dilute phase zone comprising vaporized hydrocarbon products produced from said hydrocarbon feed and a phase transition zone between said dense and dilute phase zones are present, said method comprising injecting a non-condensable medium wherein said medium is selected from the group consisting of non-condensable vapors, gases, and mixtures thereof into said dilute phase zone, to form an admixture with said vaporized hydrocarbon products wherein the dew point of the dilute phase zone is suppressed while maintaining the temperature above the suppressed dew point and wherein said dew point of said dilute phase zone will be suppressed by at least about 5xc2x0 F. (about 3xc2x0 C.) below the temperature of said dilute phase zone.
Practice of the invention comprises introducing a stream of gas or vapor, typically steam, but non-condensable at temperatures above about 705xc2x0 F. (about 374xc2x0 C.) into the dilute phase zone to form an admixture with the product hydrocarbon vapor.